1. Field of the Invention
The present invention relates to random access channel (RACH) procedure in a cellular communications network, and in particular to a method and apparatus for detecting a RACH preamble signal. While it is described below in the context of a long term evolution (LTE) type cellular network for illustrative purposes and since it happens to be well suited to that context, those skilled in the art will recognise that the invention disclosed herein can also be applied to various other types of cellular networks.
2. Discussion of the Related Art
A universal mobile telecommunications system (UMTS) is a 3rd Generation (3G) asynchronous mobile communication system operating in Wideband Code Division Multiple Access (WCDMA) based on a European standard known as Global System for Mobile Communications (GSM), and general packet radio services (GPRS) The LTE of UMTS is under discussion by the 3rd generation partnership project (3GPP) that standardised UMTS.
3GPP LTE is a technology for enabling high-speed packet communications. Many schemes have been proposed for the LTE objective including those that aim to reduce user and provider costs, improve service quality, and expand and improve coverage and system capacity. 3G LTE requires reduced cost per bit, increased service availability, flexible use of a frequency band, a simple structure, an open interface, and adequate power consumption of a terminal as an upper-level requirement.
FIG. 1 is a block diagram illustrating a network structure of an evolved universal mobile telecommunication system (E-UMTS). The E-UMTS may be also referred to as an LTE system. The communication network is widely deployed to provide a variety of communication services such as voice and packet data.
As illustrated in FIG. 1, the E-UMTS network includes an evolved UMTS terrestrial radio access network (E-UTRAN) and an evolved packet core (EPC) and one or more user equipments (UEs) 101. The E-UTRAN may include one or more evolved NodeBs (eNodeB, or eNB) 103, and a plurality of UEs 101 may be located in one cell. One or more E-UTRAN mobility management entity (MME)/system architecture evolution (SAE) gateways 105 may be positioned at the end of the network and connected to an external network.
As used herein, “downlink” refers to communication from an eNodeB 103 to a UE 101, and “uplink” refers to communication from the UE 101 to an eNodeB 103. UE 101 refers to communication equipment carried by a user and may be also be referred to as a mobile station (MS), a user terminal (UT), a subscriber station (SS) or a wireless device.
An eNodeB 103 provides end points of a user plane and a control plane to the UE 101. MME/SAE gateway 105 provides an end point of a session and mobility management function for UE 101. The eNodeB 103 and the MME/SAE gateway 105 may be connected via an S1 interface.
The eNodeB 103 is generally a fixed station that communicates with a UE 101, and may also be referred to as a base station (IBS), a network entity or an access point. One eNodeB 103 may be deployed per cell. An interface for transmitting user traffic or control traffic may be used between eNodeBs 103.
The MME provides various functions including distribution of paging messages to eNodeBs 103, security control, idle state mobility control, SAE bearer control, and ciphering and integrity protection of non-access stratum (NAS) signalling. The SAE gateway host provides assorted functions including termination of U-plane packets for paging reasons, and switching of the U-plane to support UE mobility. For clarity, MME/SAE gateway 105 will be referred to herein simply as a “gateway,” but it is understood that this entity includes both an MME and an SAE gateway.
A plurality of nodes may be connected between the eNodeB 103 and the gateway 105 via the S1 interface. The eNodeBs 103 may be connected to each other via an X2 interface and neighbouring eNodeBs may have a meshed network structure that has the X2 interface.
FIG. 2(a) is a block diagram depicting an architecture of a typical E-UTRAN and a typical EPC. As illustrated, eNodeB 103 may perform functions of selection for gateway 105, routing toward the gateway during a radio resource control (RRC) activation, scheduling and transmitting of paging messages, scheduling and transmitting of broadcast channel (BCCH) information, dynamic allocation of resources to UEs 101 in both uplink and downlink, configuration and provisioning of eNodeB measurements, radio bearer control, radio admission control (RAC), and connection mobility control in LTE_ACTIVE state. In the EPC, and as noted above, gateway 105 may perform functions of paging origination, LTE-IDLE state management, ciphering of the user plane, system architecture evolution (SAE) bearer control, and ciphering and integrity protection of non-access stratum (NAS) signalling.
FIGS. 2(b) and 2(c) are block diagrams depicting the user-plane protocol and the control-plane protocol stack for the E-UMTS. As illustrated, the protocol layers may be divided into a first layer (L1), a second layer (L2) and a third layer (L3) based upon the three lower layers of an open system interconnection (OSI) standard model that is well-known in the art of communication systems.
The physical layer, the first layer (L1), provides an information transmission service to an upper layer by using a physical channel. The physical layer is connected with a medium access control (MAC) layer located at a higher level through a transport channel, and data between the MAC layer and the physical layer is transferred via the transport channel. Between different physical layers, namely, between physical layers of a transmission side and a reception side, data is transferred via the physical channel.
The MAC layer of Layer 2 (L2) provides services to a radio link control (RLC) layer (which is a higher layer) via a logical channel. The RLC layer of Layer 2 (L2) supports the transmission of data with reliability. It should be noted that the RLC layer illustrated in FIGS. 2(b) and 2(c) is depicted because if the RLC functions are implemented in and performed by the MAC layer, the RLC layer itself is not required. The packet data convergence protocol (PDCP) layer of Layer 2 (L2) performs a header compression function that reduces unnecessary control information such that data being transmitted by employing Internet protocol (IP) packets, such as IPv4 or IPv6, can be efficiently sent over a radio (wireless) interface that has a relatively small bandwidth.
A radio resource control (RRC) layer located at the lowest portion of the third layer (L3) is only defined in the control plane and controls logical channels, transport channels and the physical channels in relation to the configuration, reconfiguration, and release of the radio bearers (RBs). Here, the RB signifies a service provided by the second layer (L2) for data transmission between the terminal and the E-UTRAN.
As illustrated in FIG. 2(b), the RLC and MAC layers (terminated in an eNodeB 103 on the network side) may perform functions such as scheduling, automatic repeat request (ARQ), and hybrid automatic repeat request (HARQ). The PDCP layer (terminated in eNodeB 103 on the network side) may perform the user plane functions such as header compression, integrity protection, and ciphering.
As illustrated in FIG. 2(c), the RLC and MAC layers (terminated in an eNodeB 103 on the network side) perform the same functions as for the control plane. As illustrated, the RRC layer (terminated in an eNodeB 103 on the network side) may perform functions such as broadcasting, paging, RRC connection management, RB control, mobility functions, and UE measurement reporting and controlling. The NAS control protocol (terminated in the MME of gateway 105 on the network side) may perform functions such as an SAE bearer management, authentication, LTE_IDLE mobility handling, paging origination in LTE_IDLE, and security control for the signalling between the gateway and UE 101.
The NAS control protocol may use three different states; first, a LTE_DETACHED state if there is no RRC entity; second, a LTE_IDLE state if there is no RRC connection while storing minimal UE information; and third, an LTE_ACTIVE state if the RRC connection is established. Also, the RRC state may be divided into two different states such as a RRC_IDLE and a RRC_CONNECTED.
In RRC_IDLE state, the UE 101 may receive broadcasts of system information and paging information while the UE 101 specifies a discontinuous reception (DRX) configured by NAS, and the UE has been allocated an identification (ID) which uniquely identifies the UE in a tracking area. Also, in RRC-IDLE state, no RRC context is stored in the eNodeB 103.
In RRC_CONNECTED state, the UE 101 has an E-UTRAN RRC connection and a context in the E-UTRAN, such that transmitting and/or receiving data to/from the network (eNodeB) becomes possible. Also, the UE 101 can report channel quality information and feedback information to the eNodeB 103.
In RRC_CONNECTED state, the E-UTRAN knows the cell to which the UE 101 belongs. Therefore, the network can transmit and/or receive data to/from the UE 101, the network can control mobility (handover) of the UE 101, and the network can perform cell measurements for a neighbouring cell.
In RRC_IDLE mode, the UE 101 specifies the paging discontinuous reception (DRX) cycle. Specifically, the UE 101 monitors a paging signal at a specific paging occasion of every UE specific paging DRX cycle.
The procedure where a UE sends a first message to a network is commonly referred to as initial access. In most systems the initial access is initiated by a UE transmitting a connection request message including the reason of the request, and receiving an answer from the network indicating the allocation of radio resources for the requested reason.
In 3GPP TS 25.331 there are several reasons, referred to as establishment causes, for sending a connection request message. Establishment causes include: originating conversational/streaming/interactive/background/subscribed traffic call, terminating conversational/streaming/interactive/background call, emergency call, inter radio access technology (RAT) cell re-selection, inter-RAT cell change order, registration, detach, originating high/low priority signalling, call re-establishment and terminating high/low priority signalling.
An “originating call” establishment indicates that the UE 101 wishes to setup a connection, for instance a speech connection. A “terminating call” establishment indicates that the UE 101 answers to paging. A “registration” establishment indicates that the user wants to register only to the network.
To initiate access to the network a random access procedure is used. The physical random access transmission is under the control of higher layer protocol which performs some important functions related to priority and load control. These procedures differ in detail but GSM, UMTS and LTE radio systems have some similarities between them.
In the random access procedure the UE 101 randomly selects an access resource and transmits a RACH preamble to the network. A preamble is a short signal that is sent before the transmission of the RACH connection request message. The UE 101 can repeatedly transmit the preamble by increasing the transmission power each time the preamble is sent until the network indicates the detection of the preamble. Then the message part is sent at the level of power equal to the last preamble transmission power plus an offset signalled by the network.
A random access channel (RACH) is a common physical channel dedicated to the random access procedure. Uplink transmissions are generally initiated through a RACH. A UE sending data on a RACH has not yet been identified by the target eNB. RACH is typically a common uplink common channel used for transmitting control information and user data and can be sued for low data transmission from the higher layer. Such a channel is said to be contention-based since many users can attempt to access the same base station simultaneously, leading to collisions. A RACH channel can be used for several purposes. For example the RACH can be used to access the network, to request resources, to carry control information, to adjust the time offset of the uplink in order to obtain uplink synchronisation, to adjust the transmitted power, etc.
A random access procedure can be launched by the UE or the eNodeB. It may, for instance, be triggered by the following events:                a UE switches from power-off to power-on and needs to be registered to the network.        a UE is not time-synchronized with a eNodeB and starts transmitting data (for instance the user calls).        a eNodeB starts transmitting data to the UE but they are not synchronized (for instance the user receives a call).        a eNodeB measures a delay of the received signal from the UE (for instance the user is moving and has lost synchronization).        a UE is moving from one cell to another and needs to be time-synchronized with a different target eNodeB than the serving eNodeB it is registered to (handover).        
FIG. 3 illustrates the sequences of messages and responses exchanged between a user equipment UE 101 and a base station eNB 103 in a typical RACH procedure.
Firstly the UE 101 retrieves information transmitted periodically from eNB 103 on a downlink broadcast channel (BCH) and selects a preamble signature, a RACH time slot and a frequency band. The preamble signature is chosen by the UE 101 from among a set of signatures known by the eNB 103. The UE 101 generates a single random access burst containing the chosen signature and transmits it to the eNB 103 over the selected time slot at the selected frequency in message 1.
The random access burst consists of a cyclic prefix, a preamble, and a guard time during which nothing is transmitted as illustrated in FIG. 4. OP denotes cyclic prefix, GT denotes guard time, RTD denotes round trip delay and TTI denotes transmission time interval.
The preamble is sent before a RACH connection request and indicates that the UE is about to transmit data. The random access burst is transmitted during one subframe. While the UE is not synchronized in the time domain, its random access burst may overlap with the next subframe and generate interference. A guard time may thus be added to combat interference. The guard time (GT) should be at least equal to the round-trip delay at the cell edge.
During the random access procedure, several users share the same channel. In order to address contention issues they are distinguishable by virtue of orthogonal sequences. These sequences are seen as the UE preamble signatures that can be transmitted simultaneously by a number of UEs. A collision occurs whenever several users choose the same signature and send it within the same time and frequency resources.
The eNB 103 monitors the current RACH slot in an attempt to detect preambles transmitted from UEs in the corresponding cell.
On reception of a signal the eNB 103 correlates the received signal in the RACH sub-frame with all possible signatures. Detection of the preamble can be either performed in the time domain or in the frequency domain. A detection variable is computed for each signature. If the detection variable exceeds a certain threshold, the preamble is considered detected.
The eNB 103 sends a random access response to acknowledge the successfully detected preambles in message 2. This message is sent on a dedicated downlink channel and uses the detected signature. It contains a timing advance command, a power-control command. If the procedure is contention-free then the UE and the eNodeB are thereby aligned in the time domain.
If the UE 101 receives a response from the eNB 103 the UE 101 decodes the response and adapts its transmission timing, and its transmission power if the response contains power control information. The UE 101 then sends a resource request message—message 3—on a dedicated uplink channel. In this message, the UE requests bandwidth and time resources to transmit data and it also indicates a UE-specific identifier. If the UE requests resources, the UE 101 uses a specific ID in the message to resolve contentions. Then the UE monitors a specified downlink channel for response from the eNB. In the case of a positive resource grant, the subsequent transmissions are carried out as normal.
The eNB attempts to resolve any contentions. If the eNB 103 receives a resource request with a UE-specific ID or signature the eNB 103 checks how many UEs were detected with the same signature and resolves any possible contentions. If the preamble sent by UE 101 was in collision with a preamble from another UE, the eNB 103 sends a contention resolution message—message 4—to give the command to UE 101 to re-start the RACH procedure. If on the other hand the UE 101 was not in collision, the eNB sends a resource assignment message—message 5. In this case the subsequent transmissions are carried out as usual. The eNB 103 identifies the UE 101 and assigns resources according to the scheduling rules applied.
In the random access response, message 2, the UE may receive an ACK signal from the eNB to indicate that a message can be sent, a NACK signal indicating that the preamble was detected but a message cannot to be sent, or no response indicating that the preamble was not detected.
In the case where UE 101 receives no response indicating that a preamble has not been detected at the first attempt, the UE 101 waits for the next RACH slot to send another preamble. The preamble signal-to-noise ratio (SNR) is relatively low compared to data SNR owing to the length of the zero-correlation sequences. Given that the random access channel does not generate much interference, the UE can afford to increase the transmission power by a few decibels (dB) at the second attempt to prevent consecutive failures (power ramping method). A too long delay is not desirable, especially in the case of handovers. The UE 101 repeatedly transmits the preamble by increasing the transmission power every time the preamble is sent until the network indicates the detection of the preamble. The procedure is exited after a certain number of failures. If a preamble is successfully transmitted the message part is generally sent at the level of power equal to the last preamble transmission power plus an offset signaled by the network.
One reason why a preamble may not be detected on a first attempt may be severe channel attenuation. If the next RACH slot occupies the same position in frequency domain then the channel is likely to fade again and the preamble undetected. Frequency hopping for RACH has been introduced to increase the detection performance of preamble retransmission by means of frequency diversity. A drawback of frequency hopping is that it requires more overhead in broadcast information and may introduce delay in the case of handovers.
A particular drawback of frequency hopping is that it can not be performed within a 1.4 MHz bandwidth since the size of the transmission bandwidth configuration (6 RBs) is the same as the PRACH transmission size.
A further drawback arises in the case of a handover (HO) with a 20 ms RACH period. When a UE performs a HO from a serving cell to a target cell it needs to know which RACH frequency to use. A prior art solution proposes that the frequency hopping pattern is based on SFN (System Frame Number). By knowing the SFN the UE can then determine which frequency is reserved for random access. There are different possibilities for an UE to obtain the SFN of the target cell:
The SFN of the target cell may be obtained through dedicated signaling (a handover command message) by the serving cell. This assumes some information exchange between eNBs, in order to match the SFN of the serving cell with the beginning of the frequency hopping pattern (frequency hopping period) in the target cell. Knowing the SFN an UE would be able to derive the frequency for RACH transmission in a target cell based on the SFN of serving cell from which it performs HO. This solution does not require knowing which radio frames allow RACH in the target cell in the case of a 20 ms random access period. Signaling overhead and lack of SFN accuracy in unsynchronised networks are however the main drawbacks of this approach.
The SFN of the target cell may be obtained by decoding the P-BCH (Physical Broadcast Channel) from the target cell. However the 40 ms P-BCH TTI (Physical Broadcast Channel transmission time interval) can lead to unacceptable HO interruption time.
An object of the present invention is to provide an alternative method of increasing the probability of detection of a RACH preamble and to thereby improve the RACH procedure.